1. Field of the Invention
The present invention relates to a vehicular lamp unit that creates a predetermined light distribution pattern with a light radiated from its semiconductor light-emitting element.
2. Description of the Related Art
Predetermined configuration and illuminance distribution are required for a light distribution pattern of a vehicular headlamp. In other words, vehicular headlamps need to create a low-beam distribution pattern that has high forward visibility and in which a contrast boundary (horizontal cut-off line) is created by removing a part of the radiated light.
In recent years, semiconductor light-emitting elements, such as light-emitting diodes (LED), have technically improved remarkably so that they have an enhanced luminance and low power consumption. As a result, they are expected to be used as a vehicular light source of next-generation.
Conventionally, a vehicular lamp unit that employs this type of LED includes a plurality of optical units that project the shape of an LED chip so as to create a predetermined configuration and illuminance distribution by way of overlapping the distributed lights.
U.S. Pat. No. 7,131,758 for example, discloses a vehicular lamp unit that contains a plurality of lamp units which radiate light to create a horizontal cut-off line. In this vehicular lamp, each of the lamp units is comprised of a light source and a projection lens. The light source includes a light-emitting diode in which a light-emitting chip configured in a rectangular shape is provided to face the front with one side of the rectangular chip horizontally extending, and the projection lens is provided in front of the light source to project the light source image to the front of the lamp as an inverted image. With this structure, the inverted image of the light source projected to the front of the lamp is an image that has a substantially rectangular shape with an upper edge thereof extending substantially horizontally. In this construction, the vehicular lamp unit employing LEDs controls the predetermined light distribution pattern and illuminance distribution.
In the above-described vehicular lamp unit that employs conventional LEDs, a plurality of optical units that project the shape of the LED chip are provided, and by way of allowing the light distributed to overlap each other, a predetermined shape and illuminance distribution is obtained. However, this results in an enlargement of lamp unit in size, causing high production cost.
Accordingly, what has been long awaited so as to be used practically is a vehicular lamp unit in which a single unit that directly projects an image of LED chip creates a predetermined shape and illuminance distribution.
However, LED chips emit light at a substantially constant luminance across the entire light-emitting region. In a direct optical system that directly projects the image of LED chip, a low-beam distribution pattern P as shown in FIG. 9A is created by controlling the image by a projection lens to horizontally and vertically enlarge the image of the LED chip. However, since the image of the LED chip at uniform luminance per unit area is projected, luminance in a part of the pattern center portion P1 which is required to obtain particularly high forward visibility declines. Accordingly, the problem is that uneven illuminance gradient in the light distribution pattern occurs as illustrated in FIG. 9B.
In addition to high luminance, recent semiconductor light-emitting elements emit white light, and such semiconductor light-emitting elements are ready for practical use, and it is more and more expected that semiconductor light-emitting elements be used as a vehicular light source in place of conventional halogen lamps. However, when light distribution and illuminance are considered, semiconductor light-emitting elements have disadvantages that there is a variation in the color of emitted light and they tend to emit bluish white or yellowish light.